​Statement from Assistant Commissioner Neil Basu, National Lead for Counter Terrorism Policing in the UK:

Statement from Assistant Commissioner Neil Basu, National Lead for Counter Terrorism Policing in the UK:

"I would like to begin by offering my deepest condolences to the family of Dawn Sturgess, who sadly died on Sunday.

"I would also like to offer my sincere thanks to the people of Salisbury and Amesbury for the tremendous support and understanding that you have shown to both our colleagues from Wiltshire Police, but also to the hundreds of officers from Counter Terrorism Policing who have been investigating how five people came to be poisoned by a deadly nerve agent since March 4.

"As some of you may know, my role is the National lead for Counter Terrorism Policing in the UK. I run London's Counter Terrorism Operations and I coordinate the work of the UK's eight other regional Counter Terrorism Units in England and Wales on behalf of my fellow chief constables.

"In March this year, we launched an investigation after Sergei and Yulia Skripal were both poisoned with the nerve agent Novichok.

"It was led by the Counter Terrorism Policing South East Unit, answerable to both the South West Chief Constables, as well as myself, and the investigation was supported by Scotland Yard, along with every other Counter Terrorism Unit in the country.

"A huge multi-agency response was set up in the wake of that incident, to identify and establish who was responsible for such a despicable act on British soil, and to identify anywhere that could have been contaminated by that nerve agent and make it safe for those who visit, work or live in Salisbury and the surrounding area.

"I would love to be able to stand here and say how we have identified and caught those responsible and how we are absolutely certain there are no traces of nerve agent left anywhere in the county. The brutal reality, however, is that I cannot offer you any such assurances or guarantees at this time.

"Late on Tuesday last week, we received that dreadful news that two more people - Dawn Sturgess and Charlie Rowley - had fallen ill as a result of exposure to Novichok.

"On Sunday, we were told that Dawn had tragically died. Our thoughts are with Dawn's family during what must be an unimaginable and devastating time for them.

"Dawn's partner, Charlie, remains in hospital, although we have received encouraging news today that Charlie's condition is improving and that he has regained consciousness.

"As I've said, the role of Counter Terrorism Policing is to now investigate these two incidents - the latest of which is being treated as murder and to try to identify and bring those responsible to justice.

"At this stage, we cannot say with certainty that both the incident in March and this latest incident are linked. Clearly, this is our main line of enquiry, but our investigation must be led by the evidence available and the facts alone.

"I would need a forensic link to be definitive, but this is a very rare substance banned by the international community and for there to be two separate distinct incidents in one, small English county is implausible to say the least.

"That said, scientists at the Defence, Science and Technology Laboratory at Porton Down, probably the leading scientists in the world, will work hard to establish if the nerve agents from the two incidents are from the same batch, but as with any police investigation, we cannot make assumptions and we have to follow where the evidence takes us.

"It may be that it will never be possible to establish such a definitive link, but DSTL are already crystal clear that it is the same nerve agent type - a Novichok - one that killed Dawn and poisoned Sergei and Yulia Skripal as well as Nick Bailey and Charlie Rowley.

"In relation to the latest incident in Amesbury, our priority is to identify how and when both Dawn and Charlie came to be contaminated. We have around 100 counter terrorism detectives working around the clock on this.

"You will have seen a number of cordoned-off sites in Salisbury and Amesbury. We have cordoned these sites off as they have been identified as places that Dawn and Charlie visited in the period prior to them falling ill.

"Of the various sites that have been cordoned off, our focus - at this time - is on Dawn's address within John Baker House in Salisbury, Charlie's address in Muggleton Road, Amesbury and an area in the Queen Elizabeth Gardens in Salisbury.

"We believe that Dawn and Charlie handled some kind of container which the nerve agent was in, and we're focusing our efforts on finding this container.

"Specialist officers are carrying out painstaking searches, but as I'm sure you will appreciate, this work is made all the more difficult as they have to carry out their activity in protective equipment, which significantly impacts the speed at which they can work.

"This, coupled with the extreme heat they have been working in, has proved extremely challenging for those carrying out this crucial work, but I know the officers are doing everything they possibly can to progress this as quickly as they can.

"However, we expect this work to take several weeks, if not months, but we simply cannot take any risks, both with our officers' safety, and with the safety of the public.

"I know this has had a great impact on those living and working in the affected areas, and I can assure you that we are working as fast as we can, but we have to be absolutely meticulous in our work.

"The investigation has pieced together a timeline of Dawn and Charlie's movements prior to them falling ill, which I am sure many of you are aware of.

"One important point I want to stress is that we have not found any evidence that either Dawn or Charlie visited any of the sites that underwent decontamination following the attack on the Skripals in March.

"You may also be aware that we recovered the bus upon which they travelled on Friday night, 29 June, as well as a red Transit van that Charlie travelled in on Saturday 30 June.

"Tests on the bus have shown no trace of the nerve agent. The van is still undergoing examination, but four other men who had been in the van prior to its seizure by police have all been contacted and preliminary tests show that they are clear and have no symptoms.

"Officers have also identified and spoken to a number of people who we know were with Dawn and Charlie before they became ill.

"Clearly, everyone Dawn and Charlie were in contact with prior to them falling ill is a focus for our inquiry and the more we know about their exact movements, the better.

"I am therefore appealing again to anyone who may have been with them or seen them in the period before they fell ill to contact us on 0800 789 321.

"We continue to work extremely closely with our public health colleagues and scientific experts to monitor and assess the ongoing levels of risk to the public.

"As the investigation progresses I can assure you that we are sharing everything we possibly can about the investigation with our colleagues so that they can, in turn, assess the risk and provide the best possible advice to the public.

"Finally, I would like to reiterate my thanks to you, the public of Salisbury and Amesbury, for your ongoing support while this important work continues. You have shown and continue to show resilience and patience."